Study Of Temperature Difference Air Flow Sensor Used In Automobiles

The key to Electronic Fuel Injection System of automobile engine is the accurate measurement of air mass flux in manifold in order to maintain an optimal air-fuel ratio. For many air flow meters, the thermal film based air flow meter develops very rapidly and begins to be applied to automobile widely, due to its many advantages such as no mechanical transmission, no pressure compensation, quick response and high sensitivity. In this thesis, the main object of this study is the temperature difference thermal film based flow sensor. The main research contents of this thesis are as follows:Due to the fact that the air flow velocity range in manifold is inconsistent with the measurement range of thermal film flow sensor, and the flow condition in manifold is turbulent, which does not meet the measurement conditions of the thermal flow sensor. An especially small sampling channel is designed inside the manifold in this research, so that the laminar flow condition can be maintained throughout the measurement range in the small sampling channel. Through measurement of flow velocity in small sampling channel, the air mass flux in manifold can be obtained. By adjusting the structure parameter of small sampling channel, the flow velocity range in small sampling channel is maintained to be consistent with the measurement range of thermal film flow sensor.The traditional thermal film flow sensor is studied. Extensive CFD simulations using the software package FLUENT are performed throughout the design process, and the temperature distribution, measurement range and measurement precision are obtained. The optimal resistor layouts and structure parameter are obtained. The maximal measurable velocity is 5m/s. The temperature difference membrane flow sensor chip is fabricated by semiconductor technology. The heater temperature is maintained at a constants temperature by a Wheatstone bridge with the heater resistor as a bridge arm, and signal voltage is obtained by another Wheatstone bridge. The simulation is verified to be appropriate by experiment. When the air mass flux in manifold is 8-370 kg/h, the output signal voltage is 0V-3.1V after magnification. The effect of heater temperature and heater width on measurement precision and measurement range is investigated. In this study, a second difference scheme for the temperature is proposed. The maximal measurement range is broden largely, but the measurement precision decreases remarkably.A type of flow sensor based on glass substrate is proposed. Due to the low thermal conductivity of glass, the membrane does not need to be structured. Therefore the fabrication technology is simple, and it can work under large fluid press. On the other hand, the consumption of this flow sensor is large, and the measurement precision is low. The temperature field is simulated by software FLUENT under different conditions, and the temperature difference is calculated for different positions on the glass surface, the relationship of output signal voltage and income mass flux is obtained. The flow sensor based on glass is fabricated by semiconductor technology, and all of this simulation is verified to be correct. The maximal measurable velocity can reach 10m/s. When the intake air mass flux is 8～370Kg/h, the signal voltage is 0V-1.4V.A new type of novel flow sensor integrated with a micro channel is proposed. The uniqueness of this flow sensor is that there is thermal convection effect on both sides of diaphragm by flow guide of a micro channel, which results in a more sensitive temperature distribution. The temperature characteristics of the novel flow sensor are simulated at different flow rates, and the optimization positions of the test resistor pair on the membrane are obtained from the simulations. Compared with traditional flow sensors, this flow sensor has higher measurement precision, and the maximum output signal voltage improves 25%. When the air mass flux in manifold is 370 kg/h, the highest signal voltage is 3.86V. However, the measurement range is narrower than that of traditional flow sensor, and the maximal measurable velocity is 3m/s.Dynamic response properties of all types of flow sensors are studied. In the manifold of engine, flow is unsteady and periodical variety. The dynamic properties of three flow sensors have been simulated, and the main effects of dynamic response are the times of flow field and temperature field achieving a balance and heater temperature compensation time. Under positive step pulse flow, the response times are 1.4ms, 2.5ms and 1.2ms respectively for three flow sensors, and under negative step pulse flow, the response times are 1.5ms, 2.5ms and 1.5ms respectively for three flow sensors.The effects of ambient temperature on measurement signal of flow sensors are studied. With ambient temperature increasing, signal voltage of flow sensors decrease remarkably, which will bring a lager deviation. With ambient temperature decreasing, the saturation effect of temperature difference becomes more remarkable, and the measurement range becomes narrower. In this research, an ambient temperature compensation method is proposed. On the front of flow chip, ambient temperature is tested by a resistor R_k, and it is connected to Wheatstone bridge as a part of bridge arm. A constant temperature difference is maintained between heater temperature and ambience temperature. After this the signal deviations of flow sensor is decreased largely. There is still a small deviation, due to the material parameter varying with ambient temperature varying.